By Kim Eun-jung SEOUL, Feb. 12 (Yonhap) -- North Korea is believed to have detonated a nuclear device with a yield of 6-7 kilotons, which is more powerful than bombs used in previous atomic tests, Seoul's defense ministry said Tuesday.

Hours after 4.9-magnitude seismic tremors with unusual sound waves were detected near the North's nuclear test site in its northeastern tip shortly before noon, Pyongyang's state media announced it has successfully tested a "smaller and lighter nuclear warhead" at an underground site.

Based on the seismic tremor, the military estimated the nuclear test had a yield of 6-7 kilotons, which is more powerful than previous tests but weaker than the bombs detonated in Japan during World War II, ministry spokesman Kim Min-seok said, noting the blast has been deemed "destructive."

"In-depth analysis is under way to determine the strength of the nuclear blast," Kim said during a briefing. "The nuclear device yield estimate can vary depending on the evaluation method."

One kiloton is equivalent to 1,000 kilograms of TNT explosives. North Korea's first nuclear test involved a weapon that had an estimated yield of about 1 kiloton, while its second may have been a 2 to 6 kiloton device, both using plutonium.

Atomic bombs dropped on Japan's Hiroshima and Nagasaki during World War II yielded a blast of about 16-21 kilotons, according to officials.

It was not immediately known whether the bomb was made of plutonium or highly enriched uranium, and it would take time to collect air samples from the test site and determine the fission material, Kim said.

If progress has been made since the isolated state revealed its uranium enrichment centrifuge plant to an American nuclear expert in November 2010, Seoul officials believe the North would be capable of testing a bomb made of uranium, which could open a second route for developing weapons.

The defense ministry immediately launched an emergency operation team, while South Korea and the United States have stepped up their joint military posture and surveillance status to handle further provocations.

"We are closely watching the current situation because the North could conduct additional nuclear tests or fire off missiles," Kim said in a briefing. "South Korea and the United States are mobilizing all available intelligence sources to monitor (North Korea)."

Later in the day, Defense Minister Kim Kwan-jin and Joint Chiefs of Staff Chairman Jung Seung-jo held an emergency meeting with Gen. James Thurman, chief of the U.S. Forces Korea, and American ambassador Sung Kim to share intelligence and prepare measures to handle heightened tension on the Korean Peninsula.

"Defense Minister Kim and U.S. representatives shared a consensus that North Korea's nuclear test is a grave provocation that destroys peace and stabilities on the Korean Peninsula and across the world. Allies and the global community should sternly deal with it," said a statement released after the closed meeting. "South Korea and the U.S. made it clear that North Korea should take severe responsibility for all consequences caused by its nuclear test."

Seoul officials said the latest test fell short of earlier expectations as some had speculated the North may test at least a 10-kiloton nuclear device or detonate a "boosted fission weapon," which is a smaller, more sophisticated bomb made of less fissile material for a given yield. Either uranium or plutonium can be used to develop this type of bomb.

"If the nuclear test fits the standard for a high-level test pledged by North Korea, it should create a yield over 10 kilotons," a senior military official said, asking for anonymity as he is not authorized to speak to media. "Based on the explosive power, (the nuclear bomb) falls far short of a boosted fission weapon."

While many have paid keen attention to what kind of fission materials were used in the test, the U.S. military sent a WC-135W radiation detection plane, which was on standby at a Japanese Air Force base in Okinawa, to the East Sea to collect air samples leaked from the northern test site, according to military officials.

In a parliamentary defense committee meeting later in the day, defense chief Kim also confirmed that a U.S. surveillance aircraft was conducting its mission to collect radioactivity in the air.

"If leaked radioactivity is collected, (the aircraft) would be able to immediately identify (fission materials)," Kim said.

While monitoring seismic tremors is a quicker way to detect a nuclear explosion, radioactivity detection is a more comprehensive method. However, this method is also complicated, as sample collection could take days and is only reliable if enough material is leaked from the underground test site and if the wind blows southward during the collection period.

Radiation materials like xenon was detected after the 2006 test, but no such material was found after the 2009 test, partly due to its more elaborately designed test tunnels, according to the ministry.

Two days ahead of the atomic test, the North fired short-range missiles off its eastern coast. The KN-02 missile can reach up to 120 kilometers.

It was not immediately known why it launched the missiles at that time, but military officials speculated it was aimed at improving missile range.

The test came hours after the North's state media said members of the ruling Workers' Party Political Bureau are working on future rocket launches as part of an "all-out action of high intensity."